Stimulation of ethylene production in citrus leaf discs by mannitol

Wound ethylene formation induced in leaf tissue of citrus (Citrus sinensis [L.] Osbeck cv. "Washington Navel") by excision was significantly stimulated by mannitol after a lag period of about 6 hours. The extent of stimulation was dependent upon the concentration of mannitol (10 to 100 millimolar). This increased ethylene production was not simply due to osmotic effect or water stress as other osmoticums tested failed to exert such an effect. The stimulatory effect of mannitol resulted from both the enhancement of 1-aminocyclopropane-1-carboxylic acid (ACC) formation and the conversion of ACC to ethylene. The effect on the latter step was particularly pronounced in aged discs. The use of labeled mannitol showed that it was taken up by the leaf discs, utilized for respiration, and metabolized to sucrose, but no radioactivity was detected in the ethylene.